While the sunlight incident on the earth has significant potential to match our total world oil reserve of ˜3 trillion barrels with 1.5 days of irradiation, the solar approach currently supplies only 0.015% of electricity globally. The bottleneck in the adoption of large scale solar to electrical energy conversion is the low efficiency of the photovoltaic (PV) conversion and high manufacturing cost of solar cells. All of these undesirably stall the lowering of solar cell prices from its current level of >$0.3-4/kW-hr to the market acceptance level of ˜$0.1/kW-hr. The bottom line is that the solar-panel production cost has to be significantly less than the current cost of about $1/W. With the technology advancement in the production of low-cost and large-area solar cells, novel applications such as wearable solar cells fabricated on polymer or other flexible materials will further expand the solar cell market well beyond that presently envisaged.
Among many solar absorbing semiconducting materials, the absorption spectrum of copper indium gallium diselenide (CIGS) thin films matches the solar spectrum when the stoichiometry of Cu(InxGa1-x)Se2, with x ranging about 0.6 to 0.8, is reached. They have great potentials to reach very high PV conversion in solar cells because of this behaviour. CIGS will therefore play a vital role in solar cell production. In fact, conversion efficiencies of ˜20% has already been demonstrated with CIGS thin film solar cells in research laboratories. Following the prevalent technology in producing compound semiconductors for optoelectronics in the current market, nearly all global players in CIGS business produce CIGS by the high vacuum techniques of evaporation or sputtering. However, both the capital and operation costs involved in these methods of fabrication are high, the throughput is limited and is not conducive to size-scalable production as would be required for efficient commercialization of these films due to the limited size of the vacuum chambers used for depositing these films.
Another technical problem is the control of the formation of stoichiometric CIGS films and proper grain properties in these films required for this material to be useful in large scale solar cell applications. Due to the relatively high vapor pressure of selenium even in a moderately elevated temperature, as-deposited CIGS films often do not have the optimum amount of selenium. The prevalent approach to circumvent the problem is to sinter as-deposited CIGS films in the presence of a selenium vapor at a temperature in the range of 300-600° C. Although the films can be fed continuously into a space-optimized sintering chamber for throughput improvement and cost reduction, this post-deposition treatment is still an extra manufacturing step requiring vacuum technology, thermal and electrical energy, and comprehensive safety procedures due to the presence of very toxic selenium vapor. More importantly, the adoption of thermal sintering at >300° C. forfeits the opportunity of producing solar cells directly on polymers and many other potentially desirable materials.
To meet the market-driven technology requirement of efficient, economical and practical large-area production of rolls of CIGS films for CIGS solar cell fabrication, Nanosolar Inc. pioneered an ink-jet printing technology of CIGS film formation (see, e.g., U.S. Pat. No. 7,122,398 issued to K. Pichler and references cited therein). In this method, nano-particles of CIGS are chemically synthesized and suspended in a colloidal liquid (commonly referred to as a “nano-CIGS ink”) with a suitable surfactant present on the surface of each nano-particle to prevent aggregation of the particles, and with other chemical additives being present that are required for the ink-printing process. The nano-CIGS ink is then be printed and a CIGS film is formed by heat treatment of the printed CIGS film to remove the solvent, surfactant, and other chemical additives, and to sinter the nano-CIGS particles into a coherent film.
U.S. Pat. No. 7,122,398 issued to Pichler discloses that after this thermal treatment step, “the film may optionally be exposed to selenium vapor at about 300-500° C. for about 30-45 minutes to ensure the proper stoichiometry of Se in the film”. It is well known in the field that such a selenization thermal treatment also improves the grain properties and electrical properties of CIGS films (see, e.g., N. Naghavi et al., Progress in Photovoltaics: Research and Applications, 2009, 17, 1-9). Hence, the technology requirement of a low-cost and large-area deposition of stoichiometric CIGS films with no requirement of any post-deposition heat treatment has not yet been fulfilled.
While it is well known that many electrically conductive materials can be deposited in a large area at low cost using electrochemical processes, at present the electrodeposition of CIGS films is very problematic for several reasons. For example, the electrodeposition of stoichiometric CIGS requires the precise solubility control of all four precursor compounds of Cu, In, Ga and Se, together with the proper controls of the electrochemical potentials for the reduction of these compounds in one single pot, with no undesirable side-products arising from other possible electrochemical reactions in the solution medium. The present inventors have confirmed experimentally that the traditional electrodeposition of CIGS from an aqueous solution is accompanied by both undesirable electrochemical reactions on the electrode prior to the deposition of CIGS and the evolution of hydrogen bubbles at the CIGS film surface during the CIGS deposition.
Nevertheless, several research groups have claimed the electrodeposition of CIGS films with the aqueous solution approach in the literature. For example, Y. P. Fu et al. (Journal of the Electrochemical Society, 2009, 156, 9 E133-E138) reported the electrodeposition of CIGS films in an aqueous solution in the presence of LiCl as a supporting electrolyte. In this publication they disclosed that post-deposition thermal sintering was still required. Furthermore, they could not produce stoichiometric thin films, as the concentration of gallium was observed to be low in the deposited films. Adding more gallium compounds to the aqueous solution was not a viable method to increase the gallium content in the resultant CIGS films because with the increase in the gallium concentration in the aqueous solution, Fu et al. noticed that the gallium reduction potential became more negative which made gallium ion deposition on the electrode more difficult. Another problem with the aqueous solution approach is that molybdenum is commonly used the electrode-contact for CIGS but molybdenum and many other metals oxidize in the aqueous cyclic voltammetry process of CIGS electrodeposition.
Recently Lai et al. (Electrochimica Acta 2009, 54, 3004-3010) reported a one-step electrodeposition process of CIGS film formation in a water-dimethylformamide (DMF) solution. In this case, the co-electrodeposition of the four elements Cu, In, Ga, Se was still difficult due to the huge difference in their reduction potentials in this solution. To overcome this problem, Lai et al. added a complexing agent into the water-DMF bath. In this publication, Lai et al. also elaborated on the aforementioned problem of hydrogen evolution. In essence, Lai et al. were not able to demonstrate the preparation of stoichiometric CIGS films.
In another approach, Kois et al. (Thin Solid Films 2008, 516, 5948-5952) reported the fabrication on CIGS films from thiocyanate complex electrolytes. Once again, they reported the necessity of post-deposition thermal selenization. Their report also shows that their CIGS films were deficient in Ga.
In yet another approach, Long and coworkers (Journal of Physics: Conference Series 2009, 152, 012074) reported the preparation of CIGS films by a one-step electrodeposition in an alcohol solution. Again, a post-deposition thermal sintering process at 550° C. for 30 min was required. Moreover, the report shows that the resultant CIGS films were still observed to be deficient in copper.
A method of electrochemical deposition of CIGS in a non-aqueous solution such as an ionic liquid was recently disclosed by Peter and coworkers (“Electrochemical Deposition of CIGS by Means of Room Temperature Ionic Liquids”, Thin Solid Films, 2007, 515, 5899-5903). This publication describes the deposition processes for the preparation of Cu—In—Se and Cu—In—Ga—Se precursor films, which were converted to stoichiometric CIS and CIGS films respectively with a post-deposition thermal selenization at 500° C. for 30 min. The present inventors have repeated the CIGS deposition process disclosed by Peter and coworkers, by following the same bath compositions and applied potential conditions, and found that the disclosed electrodeposition process alone did not produce stoichiometric CIGS films. Typically, the as-deposited CIGS films prepared by the disclosed process have very low selenium content, and show a very poor morphology homogeneity and weak adhesion to molybdenum substrates. The present inventors suspect that this is also the reason for adopting the post-deposition thermal selenization step required by Peter and coworkers to convert their Se-deficient films to stoichiometric CIGS films.
Thus, as discussed above, while there have been recent attempts by many researchers in the field to develop a methodology to electrodeposit CIGS films, there has not been any success in electrodeposition of stoichiometric CIGS films ready for solar cell fabrication with no requirement of post-deposition annealing or selenization.
What is therefore needed is an economical, reproducible electrodeposition method of producing stoichiometric CIGS films which is scalable for use in industries such as production of large surface area solar cells without the requirement of any post-deposition sintering or selenization.